The present invention relates to reactors and susceptors, and more particularly to reactors and susceptors useful in a chemical vapor deposition process occurring in a radiant absorption heater system.
Chemical vapor deposition processes are currently used to produce a variety of high purity materials and are of special utility in the production of III-V materials (such as gallium arsenide) and other epitaxial compositions used for semiconductors, ect.
The chemical vapor deposition process is typically performed in any of a variety of reactors well recognized in the art, including horizontal reactors, vertical reactors, pancake reactors, and the like. Broadly, the reactors fall into two different classifications: radiant absorption heater systems (such as cold wall reactors) and oven reactors (such as hot wall reactors). Cold wall or radiant energy absorption systems are typically preferred over hot wall reactors for many reasons including their better yields and throughputs.
In the oven reactor, all of the equipment associated with the process is heated more-or-less to the reaction temperature of the source gases, which then deposit the material to be formed not only on the intended substrate, but also, for example, on the walls of the reactor vessel, thereby wasting both energy and the source materials, creating cleaning problems and, when different source materials are to be used for successive runs, presenting contamination problems between runs. By way of contrast, in a radiant absorption heating system, radiant energy in the form of radio frequency (RF), infra-red (IR) ormicrowave energy is projected from outside the reactor vessel into the reactor vessel where it is selectively absorbed by a susceptor. As the energy is absorbed by the susceptor. the susceptor becomes hotter than the surrounding elements of the reactor vessel and consequently selectively heats up a susceptor-carried substrate for the material to be deposited. While the selective heating of the substrate relative to the other reactor vessel elements is a clear advantage of the radiant absorption heater system, economic operation of such a system clearly requires a susceptor which will efficiently use the energy being introduced. The present invention is directed to a susceptor for use in such a radiant absorption heater system.
Various considerations severely restrict the selection of material for use as a susceptor. Clearly the material must be capable of efficiently absorbing the specific form of radiant energy being introduced into the reaction vessel so as to minimize the energy input required to bring the system to, and maintain it at, its usual operating temperature (this typically being in the neighborhood of 700.degree. C. or higher for the production of epitaxial III-V materials). Obviously the material must also be chemically inert with respect to the materials it contacts, including the source gases being introduced into the reaction vessel, and must not be a source of contamination to either the source gases, the substrate or the material being produced. When heated to the temperatures typically encountered by susceptors (often in excess of 1,000.degree. C.), most materials outgas--that is, they release from within gases which act as a source of contamination for the source gases, the substrates, and the material to be deposited, thereby introducing elements foreign to the desired reaction and resulting in an inferior end-product. Finally, use of the material must be economically feasible in the quantitites required for susceptor use.
In a horizontal type reactor, the susceptor has the configuration of a box of rectangular parallelopiped and the reaction gases are directed from one end, across the full length thereof, towards the other end. The wafers to be grown are disposed on only the large upwardly-facing surface of the susceptor, typically in recesses provided for this purpose. The other large surface (i.e., the downwardly-facing one) does not carry any wafers at all and is used solely for support of the susceptor. As the four sides of the susceptor do not carry any wafers, the "utilized area" for a horizontal reactor is relatively low. For example, a susceptor having a surface area of approximately 3,528 cm.sup.2 (547.1 in..sup.2) can be used for 18 wafers of 7.62 cm (three inch) diameter, disposed in three longitudinal rows of six wafers each. The power requirement (defined as the power to bring the susceptor to reaction temperature) is 50 kilowatts, and the utilized area (defined as the surface area occupied by wafers/total susceptor surface area) is about 23%.
Actually, in a horizontal reactor the susceptor is usually tilted slightly upwardly in the direction of the gas flow (i.e., downstream) so that the rear or downstream wafers are exposed to fresh input gas. This minimizes the problem of depletion which occurs when a fair sized series of successive upstream wafers deplete the input gases of the reactive components and expose the most downstream wafers only to the relatively exhausted input gases. The tilting of the susceptor can also assist in maintaining the desired temperature profile within the reactor vessel.
Other reactor types are subject to similar deficiencies. An eight sided infrared heated vertical reactor may have almost double the wafer capacity (say about 30 such wafers), but an even lower utilized area of only about 17%. As much of the surface area of its susceptor is being heated for no functional purpose, the power requirement of this type of reactor is quite high, about 115 kilowatts. Thus, a 67% increase in the wafer capacity requires a 130% increase in the power requirement. In a radio frequency heated pancake-type reactor, a slightly enhanced capacity of 21 such wafers (relative to the horizontal reactor) provides slightly enhanced utilized area of 27%, but the reactor requires 100-115 kilowatts, at least twice as much power. Thus the need remains for a reactor which is characterized by both a large wafer capacity, and a low power requirement, in other words, a high utilized area. Accordingly, it is an object of the present invention to provide a reactor having a high utilized area so as to maximize its wafer capacity and minimize its power requirement.
Another object is to provide a horizontal reactor employing a special susceptor which enables an approximate doubling of the utilized area so as to achieve an approximate doubling of the wafer capacity without an appreciable increase in the power requirement relative to a standard susceptor.
It is also an object of the present invention to provide such a reactor which minimizes or obviates the depletion problem.